Growers of onions and leeks were dealt a blow with the loss of Totril, which had been the mainstay of weed control programmes, and now linuron which was effective against problem weeds like fat hen and field pansy. Bromoxynil, the active ingredient of Buctril, used as a replacement for Totril, is undergoing risk assessment for renewal of its approval and is at risk of withdrawal too, says agronomist Andy Richardson of the Allium & Brassica Centre.
So is chlorpropham, an essential residual on peaty soils, while chloridazon, the active substance of products such as Pyramin DF, is being withdrawn from sale this year.
However, a new pre-emergence herbicide from Bayer, based on aclonifen, is expected to be approved this year for potatoes with plans for label extensions for carrots, onions and celery.
Aclonifen was tested in SceptrePlus trials on onions last year. As an alternative to Buctril, it proved to be the only product not to cause unacceptable damage to salad onions at the rate and timings used.
Several experimental products were also tested pre-emergence and although the results suggested control was poor on the weeds prevalent at the site, they may have uses for specific weeds as part of a programme or for resistance management, for instance in controlling groundsel where growers are now highly reliant on dimethenamid-P.
With a reasonable range of residual herbicides to choose from, what brassica growers are lacking are contact-acting products, says Mr Richardson. This is especially when conditions are dry, which inhibits the effectiveness of residuals and poses a particular challenge for long-season crops such as Brussels sprouts and cabbage.
One herbicide that will be welcomed when finally approved is a Gowan Crop Protection product based on benfluralin, he says. "It is soil incorporated so works well in dry soil, so we should see really good control of polygonums, which were a particular problem for Lincolnshire growers last year. Residues data has been available since the first Sceptre trials, and it's now approved in Ireland but we're still waiting in the UK.
"A number of other products are being trialled that already hold approvals on oil-seed rape in the UK and would be useful as residuals with some contact activity," says Mr Richardson.
Cauliflower was the trial crop for SceptrePlus last year because of its sensitivity to herbicide damage, says Joe Martin. Six out of the eight treatments tested are described as “promising”
"These include some actives that we have not looked at before and are either already on the market or close to it,” he says